Datasheet

AD549
Rev. H | Page 11 of 20
The case of the AD549 is connected to Pin 8 so that it can be
bootstrapped near the input potential. This minimizes pin
leakage and input common-mode capacitance due to the case.
Guard schemes for inverting and noninverting amplifier
topologies are illustrated in Figure 28 and Figure 29.
00511-028
2
3
6
8
AD549
+
V
OUT
–
I
N
C
F
R
F
GUARD
Figure 28. Inverting Amplifier with Guard
0
0511-029
3
2
6
8
AD549
+
V
OUT
–
V
S
+
–
GUARD
R
F
R
I
Figure 29. Noninverting Amplifier with Guard
Other guidelines include keeping the circuit layout as compact
as possible and keeping the input lines short. Keeping the assembly
rigid and minimizing sources of vibration reduces triboelectric
and piezoelectric effects. All precision, high impedance circuitry
requires shielding against interference noise. Use low noise coaxial
or triaxial cables for remote connections to the input signal lines.
OFFSET NULLING
The AD549 input offset voltage can be nulled by using balance
Pin 1 and Pin 5, as shown in Figure 30. Nulling the input offset
voltage in this fashion introduces an added input offset voltage
drift component of 2.4 μV/°C per mV of nulled offset (a maxi-
mum additional drift of 0.6 μV/°C for the AD549K, 1.2 μV/°C
for the AD549L, and 2.4 μV/°C for the AD549J).
00511-030
2
3
6
5
1
7
4
AD549
+
V
OUT
–
–V
S
+
V
S
10kΩ
Figure 30. Standard Offset Null Circuit
The approach in Figure 31 can be used when the amplifier is
used as an inverter. This method introduces a small voltage
referenced to the power supplies in series with the positive
input terminal of the amplifier. The amplifier input offset
voltage drift with temperature is not affected. However,
variation of the power supply voltages causes offset shifts.
00511-031
2
3
6
AD549
+
V
OUT
–
V
I
+
–
R
F
R
I
200Ω
100kΩ
499kΩ 499kΩ
0.1µF
–V
S
+V
S
Figure 31. Alternate Offset Null Circuit for Inverter
Table 3. Insulating Materials and Characteristics
Material
Volume Resistivity
(V to CM)
Minimal
Triboelectric Effect
1
Minimal
Piezoelectric Effect
1
Resistance to
Water Absorption
1
Teflon 10
17
to 10
18
W W G
Kel-F® 10
17
to 10
18
W M G
Sapphire 10
16
to 10
18
M G G
Polyethylene 10
14
to 10
18
M G M
Polystyrene 10
12
to 10
18
W M M
Ceramic 10
12
to 10
14
W M W
Glass Epoxy 10
10
to 10
17
W M W
PVC 10
10
to 10
15
G M G
Phenolic 10
5
to 10
12
W G W
1
G: good with regard to property; M: moderate with regard to property; W: weak with regard to property.